[Python-Dev] asyncore fixes in Python 2.6 broke Zope's version of medusa

[Hrvoje Niksic]

Greg Ewing wrote:
> Antoine Pitrou wrote:
> 
>> For starters, since py3k is supposed to support non-blocking IO, why not write a
>> portable API to make a raw file or socket IO object non-blocking?
> 
> I think we need to be clearer what we mean when we talk
> about non-blocking in this context. Normally when you're
> using select/poll you *don't* make the underlying file
> descriptor non-blocking in the OS sense. The non-blockingness
> comes from the fact that you're using select/poll to make
> sure the fd is ready before you access it.
> 
> So I don't think it makes sense to talk about having a
> non-blocking API as a separate thing from a select/poll
> wrapper. The select/poll wrapper *is* the non-blocking
> API.

This is not necessarily the case.  In fact, modern sources often 
recommend using poll along with the non-blocking sockets for (among 
other things) performance reasons.  For example, when a non-blocking 
socket becomes readable, you don't read from it only once and go back to 
the event loop, you read from it in a loop until you get EAGAIN.  This 
allows for processing of fast-incoming data with fewer system calls.

Linux's select(2) man page includes a similar advice with different 
motivation:

        Under Linux, select() may report a socket file descriptor
        as "ready for reading",  while  nevertheless
        a subsequent read blocks.  This could for example
        happen when data has arrived but upon
        examination has wrong checksum and is discarded.  There may
        be other circumstances  in  which  a
        file  descriptor  is  spuriously  reported  as ready.
        Thus it may be safer to use O_NONBLOCK on
        sockets that should not block.

Even if you don't agree that using O_NONBLOCK with select/poll is the 
best approach to non-blocking, I think there is enough existing practice 
of doing this to warrant separate consideration of non-blocking sockets 
(in the OS sense) and select/poll.